Method for Fabricating Small-Scale, Curved Structures

ABSTRACT

A method is proposed for fabricating small-scale, curved structures. Firstly, a calligraphy paper or filter paper is provided. Liquid-phase material droplets are applied to calligraphy paper or filter paper to form a desired pattern at a contact angle that can form convex curved small-scale structures.

TECHNICAL FIELD

The invention is generally relevant to a method of forming micro structures, more specifically, to a method for fabricating small-scale, curved structures with a contact angle on calligraphy paper.

BACKGROUND

One of the various materials used to make cell-based assays is polydimethylsiloxane (PDMS), an inert and non-toxic silicone-based polymer. Because of its mechanical, chemical, and optical properties, PDMS has many biomedically relevant applications, including fabrication of artificial organs, prostheses, catheters, contact lenses, as well as drug delivery systems. Non-biomedical applications include microfluidic devices, microreactors, lab-on-a-chip diagnostics, soft-lithography, membranes, electrical insulators, water repellents, anti foaming agents, adhesives, protective coatings, and sealants.

Many attractive characteristics of PDMS include its chemical inertness, non-toxicity, easy-handled, and commercial availability. Many PDMS surface modification strategies have been developed, including physisorption and chemical coupling. Physisorption of materials such as surfactants (Huang, B.; et al. Science 2007, 315, 81-84) and polyelectrolytes (Liu, Y; et al. Anal. Chem. 2000, 72, 5939-5944) to the PDMS surface is driven by hydrophobic and electrostatic forces, respectively. Chemical coupling is stable but generally requires high-energy (i.e., plasma) bombardment of the PDMS surface (Donzel, C.; et al. Adv. Mater. 2001, 13, 1164).

The PDMS is commercially available from several vendors as a two-part kit containing an elastomer base and a cross-linking agent, both of which are sold in liquid form. Kits are also available in varying molecular weights and/or varying branches of the elastomer base. Polymerization is initiated by mixing the elastomer base with the cross-linking agent. The resulting rubbery solid PDMS elastomer is optically transparent and has a hydrophobic surface. Although the hydrophobic nature of PDMS is often an undesirable characteristic, it is essential for microfluidic devices with hydrophilic surfaces to allow polar liquids to pass through. Biomedical devices such as contact lenses are easily wetted to improve user's comfort. Various strategies used to obtain a hydrophilic surface in PDMS include exposure to oxygen plasma, ozone, corona discharge, and ultraviolet light. Additionally, a hydrophilic surface can be modified by physical adsorption of charged surfactants, by adding polyelectrolyte multilayers, and by using a swelling-deswelling method to entangle amphiphilic co-polymers in an organic solvent. Covalent modification of the PDMS surface requires a surface activation process, generally by an oxidation reaction followed by solvent or chemical vapor deposition of the reactive molecule. A cost-effective method is needed to render PDMS with desired hydrophilic properties but without compromised mechanical, optical, or gas permeability properties.

Obtaining a PDMS surface pattern requires the placement of a photo-mask above the surface of the functionalized PDMS substrate to enable selective functionalization of the PDMS substrate. The PDMS pattern is formed by a traditional photolithography process which requires a photo-mask, followed by an etching process. Therefore, PDMS manufacturing requires specific equipment. For example, U.S. Pat. No. 9,192,922, entitled “Method of optical fabrication of three-dimensional polymeric structures with out of plane profile control”, describes a method of manufacturing three-dimensional polymeric structures by using a photolithography process and an etching process.

To address the shortcomings of PDMS and the difficulties of making PDMS surface patterns, a method is proposed for fabricating small-scale, curved, polymeric structures.

SUMMARY OF THE INVENTION

To address the above shortcomings, a method is proposed for fabricating small-scale, curved, polymeric structures by casting and curing thermocurable PDMS in photocurable PDMS molds.

Another objective of the invention is to provide a method of fabricating small-scale, curved, polymeric structures applicable in cell-based assays, antibody-based arrays, the development of synthetic biopolymers, tissue engineering, and bio-microelectromechanical systems (Bio-MEMS).

One feature of the invention is its potential use for fabricating small-scale, curved, polymeric structures composed of a flexible material. After the desired pattern is formed by placing liquid-phase photocurable material droplets on the flexible material, the liquid-phase photocurable material droplets are cured to form convex curved small-scale structures.

Another potential application is in casting a thermocurable material on the convex surfaces of small-scale structures. Small-scale structures with concave surfaces can then be formed by curing the thermocurable material.

The material used in the proposed fabrication method is photocurable PDMS or UV crosslinkable material, and the thermocurable material is thermocurable PDMS or thermal crosslinkable material.

After forming a patterned layer on the flexible material, the next step of the method is placing the droplets of liquid-phase photocurable material.

In another embodiment, the method is used to fabricate small-scale, curved structures by using calligraphy paper or filter paper. Liquid-phase material droplets are placed on mentioned calligraphy paper or filter paper to form a desired pattern and contact angle for forming convex curved small-scale structures.

Another step in the method forms a patterned layer on calligraphy paper or filter paper before the liquid-phase material droplets are applied. The patterned layer can be formed with ink, carbon powder, toner or wax.

BRIEF DESCRIPTION OF THE DRAWINGS

The attached specifications and drawings outline the preferred embodiments of the invention, including the details of its components, characteristics and advantages.

FIG. 1 shows a patterned layer formed on the substrate in one embodiment of the invention;

FIG. 2 shows convex-curved small-scale structures formed on the substrate in one embodiment of the invention;

FIG. 3 shows a cast of a thermocurable material using a small-scale polymeric mold in one embodiment of the invention;

FIG. 4 shows the removal of the concave-curved polymeric structures from the mold in one embodiment of the invention;

FIGS. 5A-D illustrate arrays of the convex curved structures in one embodiment of the invention;

FIGS. 6A and 6C illustrate arrays of the photocurable PDMS molds in one embodiment of the invention;

FIGS. 6B and 6D illustrate arrays of the polymeric, curved thermocurable PDMS structures in one embodiment of the invention;

FIG. 7 illustrates the results of a statistical analysis of six 4×4 arrays with curved structures on paper in one embodiment of the invention;

FIG. 8 illustrates grouped data for the projection area of six 4×4 thermocurable PDMS arrays in one embodiment of the invention;

FIG. 9 illustrates grouped data for the projection area of two thermocurable PDMS arrays in one embodiment of the invention;

FIGS. 10A-B illustrate the optical properties of the polymeric, curved thermocurable PDMS structures in one embodiment of the invention;

FIGS. 11A-F illustrate the optical images of Madin-Darby canine kidney cells on one well of multiple 4×4 arrays made out of thermocurable PDMS with various culture durations in one embodiment of the invention;

FIGS. 12A-D are epi-fluorescence images of NIH-3T3 fibroblasts cultured on the concave surface of the PDMS structure after two days of culture in one embodiment of the invention;

FIGS. 13(A, B & C) show the diffusion test results for droplets on different paper types;

FIG. 14 shows the homogeneity test results for diluted wax or dye droplets on different paper types in printing volume 23 square millimeters (mm²).

FIG. 15 shows the homogeneity test results for diluted wax or dye droplets on different paper types in printing volume 51 square millimeters (mm²).

FIG. 16 shows the homogeneity test results for diluted wax or dye droplets on different paper types in printing volume 78 square millimeters (mm²).

FIG. 17 shows the ratio of hydrophilic zone volume in printing volume 23 square millimeters (mm²) on different paper types, after curing.

FIGS. 18(A & B) show micro-structure images of calligraphy paper produced by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS), respectively.

FIG. 19 shows a spectrogram for calligraphy paper produced by energy dispersive spectrometer (EDS).

FIG. 20 shows a contact angle of the droplet formed on calligraphy paper.

DETAILED DESCRIPTION

Some preferred embodiments of the invention are next described in further detail. Notably, however, the preferred embodiments are provided for illustration purposes rather than for limiting the use of the invention. The invention is also applicable in many other embodiments besides those explicitly described, and the scope of the invention is not expressly limited except as specified in the accompanying claims.

The invention provides an inexpensive but robust and easily performed approach for fabricating polymeric, curved structures on a paper/or plastic substrate at millimeter scale (or in an array format at millimeter scale) for various applications. Since this simple and inexpensive method is applicable for fabricating biomedical devices, point-of-care diagnostic systems or biomaterials for scaffolds without sophisticated facilities, it can reduce the costs of the manufacturing process (e.g., costs of materials and capital) in both developing or industrialized countries. The invention can also be used to fabricate small-scale convex structures (single or multiple structures as an array) by using a biocompatible polymer (e.g., photocurable PDMS) to produce a phase transition activated by UV light and the surface tension between this polymeric material and the substrate (e.g., an inexpensive material such as paper or plastic film). Potential applications of the invention include cell-based assays, antibody-based arrays, synthetic biopolymers, tissue engineering, and Bio-MEMS. The proposed method of fabricating small-scale, curved, polymeric structures is described further below.

Firstly, a substrate 101 is prepared and formed on (or adhered to) tape 100. Examples of substrate 101 include paper or a plastic film (smooth substrate). The preferred thickness of the thin, flexible film used as the plastic film substrate is 0.01˜0.1 centimeters to provide a flexible yet dimensionally stable substrate. The plastic film surface should be sufficiently smooth to enable a good bond to tape 100. This heat stabilization ensures that the plastic film can endure the heat cycle of the curing process without cockling or buckling. Tape 100 may be an adhesive layer (tape).

Potential plastic film materials include triacetate cellulose (TAC), polyethylene, polypropylene, poly(4-thylpentene-1-ene) polyolefin, polyimide, polyamide imide, polyamide, polyether imide, polyether ether ketone, polyketone sulfide, polyether sulfone, polysulfone, polyphenylene sulfide, polyphenylene ketone, polyethylene terephthalate ethylene glycol esters, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate glycol esters, polyacetal, polycarbonate, polyacrylate, acrylic resins, polyvinyl alcohol, polypropylene, cellulose-based plastic, epoxy resins, phenol resins, poly-norbornene, polyester, polystyrene, polyvinyl chloride, polyvinylidene chloride, and liquid crystal polymer.

Patterned layer 102 is then formed on substrate 101 to facilitate alignment during the following process. Substrate 101 can be a flexible material that costs less than the silicon wafer. In one embodiment, a material (such as ink or wax) layer is formed on the substrate 101 to form a patterned layer 102 and a non-patterned area 103, shown in FIG. 1. Patterned layer 102 is formed from material used for laser printing such as toner or carbon powder. Patterned layer 102 can be created by a standard printing process using a commercial printer or by a photomask-free process. Non-patterned area 103 may be designed in an array format at millimeter scale. The liquid-phase photocurable material (e.g., UV-activated material) droplets are applied to non-patterned area 103 on substrate 101 to form the desired pattern of liquid-phase photocurable material droplets on substrate 101. The material used to form the desired patterns on substrate 101 via patterned layer 102 can be either ink or wax. The liquid-phase photocurable material droplets with the desired patterns are then activated (cured) by exposure to a light source such as UV light. FIG. 2 shows that the convex curved small-scale structure (or multiple structures as an array) 104 is formed on substrate 101 via a combination of the surface tension between the selected substrate 101 and the photocurable material (e.g., polymeric material) and UV activation (e.g., wavelength˜365 nm). Thus, the photocurable material droplets switch from liquid phase to solid phase. The photocurable material, (e.g., UV-activated PDMS), is prepared by mixing prepolymer ((methacryloxypropyl)methylsiloxane-dimethylsiloxane) with 1˜10% (weight in weight: w/w) photoinitiator (2,2-dimethoxy-2-phenylacetophenone). Restated, the photocurable PDMS can be prepared by mixing liquid flexible material with solid powder photoinitiator.

Moreover, the shape of the convex-curved small-scale structure 104 depends on the surface tension between the photocurable material and the selected substrate 101 and on the delay between formation of the viscoelastic droplet and activation (crosslink) of photocurable PDMS under UV exposure. For example, FIGS. 5(A-C) show arrays of various sizes of curved structures 104 formed from photocurable PDMS droplets on paper. FIG. 5(D) shows an array of curved structures 104 on a plastic film in which patterns were printed with a laser printer. Scale bar is equal to 1 centimeter (cm).

Accordingly, the small-scale polymeric mold and the polymeric, curved structures can be fabricated at millimeter scale (or in array format at millimeter scale) by using a physical-based combination of the phase transition of a biocompatible polymer, e.g., photocurable PDMS, and the surface tension between this polymeric material and the substrate (i.e., papers or plastic films).

The optical properties of the resulting small-scale convex structure of the photocurable PDMS structure can be modified by adjusting the concentration, molecular weight, configuration, and hydrophobic/hydrophilic balance of the polymer additive(s). Thus, the surface tension between the photocurable PDMS (polymeric material) and the substrate may be modified to change the optical properties of the formed convex small-scale structure of photocurable PDMS. As noted above, a simple and cost-effective technique for forming PDMS is needed. The hydrophobic characteristics of PDMS can be modified by adjusting the preparation conditions and subsequent treatments and exposure environments. By affecting the surface tension between PDMS and the substrate, the hydrophobic characteristic of PDMS can vary the curvature of the PDMS structure.

Another embodiment does not require patterned layer 102. The specified patterns are formed by directly dropping/placing the liquid-phase photocurable material droplets on substrate 101. That is, the embodiment does not include the process for forming patterned layer 102. Similarly, the convex curved small-scale structure (or multiple structures as an array) 104 is formed by using UV light to activate (cure) the liquid-phase photocurable material droplets.

In yet another embodiment, convex small-scale structure 104 with the same pattern as the mold is used to fabricate PDMS-based structures via a molding process. For example, FIGS. 6(A & C) show photocurable PDMS molds with 16 and 96 curved structures 104 in 4×4 and 8×12 arrays, respectively. FIG. 3 shows a structure cast from a thermocurable material using a small-scale polymeric mold. To cross-link the thermocurable material on photocurable PDMS molds, the thermocurable material is cured at 70° C. for 2 hours. Thus, concave and curved polymeric structures 106 are obtained at millimeter scale by using the mold to cast and cure the thermocurable material (PDMS). FIG. 4 shows curved polymeric structure 106 after its removal from the mold. The bond between the curved polymeric structures 106 and the mold enables easy detachment of the curved polymeric structure 106 from the mold after curing. For example, FIGS. 6(B & D) show examples of polymeric, curved thermocurable PDMS structures 106 prepared by casting and curing thermocurable PDMS in the above two molds (scale bar=1 cm). FIG. 6(B) is an optical image of a thermal-activated PDMS array with 16 small-scale structures. This array may be used for making both cell-based assays and antibody-based arrays.

The photocurable material is photocurable PDMS or UV crosslinkable material (e.g., polyethersulfones), and the thermocurable material is thermocurable PDMS or thermal crosslinkable material (e.g., thermal crosslinkable resin or ethylene-vinyl acetate).

FIG. 7 shows the results of a statistical analysis of six 4×4 arrays with curved structures 104 on paper. The analysis was performed using ImageJ image analysis software (N=6; n=96). In each of the six test samples, the X-coordinate indicates whether the sample is concave or convex, and the Y-coordinate indicates the average projection area of each droplet. The average projection area per droplet (photocurable PDMS mold) approximates 0.0903 cm² with a standard deviation of 0.0099 cm² (11.0% error) whereas the average projection area per droplet (thermocurable PDMS structures) approximates 0.0873 cm² with a standard deviation of 0.0084 cm² (9.62% error).

FIG. 8 shows the grouped data for a projection area of six 4×4 thermocurable PDMS arrays (96 thermocurable PDMS curved structures), which represents a Gaussian distribution.

FIG. 9 shows the grouped data for the projection area of two (trials 1 and 2) thermocurable PDMS arrays (96 thermocurable PDMS curved structures) in a 96-well format (N=2; n=192). The X-coordinate indicates the projection area (cm²), and the Y-coordinate indicates the percentage (%).

FIGS. 10(A, B) show the optical properties of concave structures 106 made from thermocurable PDMS. The characters in FIG. 10(A) at the concave location are minimized by the divergence of light passing through the curved structures 106. Moreover, FIG. 11 shows the optical images of Madin-Darby canine kidney cells on one well of multiple 4×4 arrays made from thermocurable PDMS at varying durations of culture; FIGS. 11(A, B) show the initial state, FIGS. 11(C, D) show the state after 24 hours, and FIGS. 11(E, F) show the state after 48 hours (scale bar=100 μm). The left side of FIGS. 11(A, C & E) shows we made out of the photocurable PDMS mold made on paper, and the right side of FIGS. 11(B, D & F) shows wells made out of the photocurable PDMS mold made on plastic film.

FIGS. 12(A, D) show epi-fluorescence images of NIH-3T3 fibroblasts cultured on the concave PDMS structure after two days of culture (scale bar=100 μm). FIGS. 12(A) and 12(C) show actin filaments with Alexa Fluor 488 phalloidin, and FIGS. 12(B) and 12(D) show merged. epi-fluorescence images of actin filaments and nuclei of NIH-3T3 fibroblasts.

The invention is a simple and inexpensive method with many potential applications. For example, the concave structures 106 made of thermocurable PDMS have various applications in cell-based assays and antibody-based assays, such as Enzyme-linked immunosorbent assay (ELISA).

Another embodiment of the invention is a calligraphy paper-based diagnostic device. In the foregoing embodiment, the substrate 101 is a flexible material, such as paper or plastic film. In the present embodiment, the substrate is calligraphy paper or filter paper. The filter paper or calligraphy paper may be facilitated to form a convex curved small-scale structure with a contact angle. FIGS. 13(A-C) show the diffusion test results (pattern) for droplets on general paper, Xuan paper, and calligraphy paper, respectively. For example, the general paper and Xuan paper are composed of linen, and the calligraphy paper is composed of polyvinyl chloride (PVC); pore sizes range from 3 to 30 microns and thicknesses range from 0.15 to 0.3 millimeter. After applying artificial coloring Allura Red (Allura Red AC) liquid droplets of similar volume in 6×6 arrays on the three paper types, the diffusion patterns of artificial coloring Allura Red (pigment) droplets were compared. FIGS. 13(A-C) show that the circular arrays on the general paper are fully occupied by red pigment droplets, the circular arrays on the Xuan paper are partially immersed in red pigment droplets, and the circular arrays on the calligraphy paper are mostly immersed in red pigment droplets. The figures clearly show that many pigment droplets penetrate and diffuse into the calligraphy paper. Restated, within a given period, pigment droplets can spread and penetrate into calligraphy paper, in order to achieve the amount of saturation. The dripping speed of the droplets can be adjusted according to requirements.

FIGS. 14-16 further show the homogeneity test results for diluted wax or dye droplets on different paper types in different printing volumes; wherein the dye droplets are artificial coloring Allura Red (Allura Red AC) droplets. FIG. 14 shows the percentage of cover volume in printing volume 23 square millimeters (mm²) in different paper types. FIG. 15 shows the percentage of cover volume in printing volume 51 square millimeters (mm²) in different paper types. FIG. 16 shows the percentage of cover volume in printing volume 78 square millimeters (mm²) in different paper types. The homogeneity test results are obtained in successive increments. Percentage of cover volume of the diagrams can be calculated as the average of several tests. In FIGS. 14-16, the histograms from left to right indicate the percentages of cover volume for general paper (type: Whatman # 1), general paper (type: Whatman # 6), general paper (type: Whatman # 40), Xuan paper and calligraphy paper, respectively. The test results in FIGS. 14, 15 and 16, show that, (1) regardless of printing volume, the percentage of cover volume of dye droplets on calligraphy paper is substantially lower than that of other paper types (general paper, Xuan paper), and the percentage of cover volume of dye droplets on Xuan paper is slightly lower than that of general paper; (2) regardless of printing volume, the percentage of cover volume of diluted wax on calligraphy paper or general paper is insignificant while the percentage of cover volume of diluted wax on Xuan paper is lower. Based on the above experimental results, the high percentage of cover volume of diluted wax on calligraphy paper indicates that most of the diluted wax homogenously spreads in the lateral direction of the paper; the low percentage (even less than 30%) of cover volume of dye droplets on calligraphy paper indicates that most of the dye droplets homogenously spread in the direction perpendicular to the paper, and some dye droplets penetrating into the interior of calligraphy paper until a saturation status. FIG. 17 shows the ratio of hydrophilic zone volume in printing volume 23 square millimeters (mm²) on different paper types, after curing. Before curing, a diluted wax pattern is pre-printed on paper. For example, a hollow circle pattern is formed on the paper except diluted wax pattern. Diluted wax pattern is dissolved after curing treatment. In FIG. 17, the left side of each histogram indicates an area of the hollow circle pattern after the wax dissolves, and the right side of each histogram indicates an area of in which a similar volume of red ink droplets are applied in a hollow circle pattern, Histograms from left to right indicate percentages of hydrophilic zone volume on (a) general paper (type: Whatman # 1), (b) general paper (type: Whatman # 6), (c) general paper (type: Whatman # 40), (d) Xuan paper and (e) calligraphy paper. FIG. 17 shows that the percentage (even less than 30%) of hydrophilic zone volume of red ink droplets on calligraphy paper percentage is substantially lower than other paper types. This indicates that most of the red ink droplets homogenously spread in the direction perpendicular to the paper, and some d ink droplets penetrate into the interior of calligraphy paper.

FIGS. 18(A & B) show micro-structure images of calligraphy paper produced by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS), respectively. The following list shows ingredients and their percentage by weight of calligraphy paper. FIG. 19 shows a spectrogram for calligraphy paper produced by EDS.

Element Weight (%) Atomic (%) O K 82.70 92.35 Al K 2.63 1.74 Si K 2.43 1.55 Ca K 8.32 3.71 Pd L 3.92 0.66 Total 100.00

FIG. 20 shows a contact angle of droplet formed on calligraphy paper. As shown in FIG. 20, the characteristics of the calligraphy paper itself cause the droplets on the calligraphy paper to form a curved small-scale structure with a contact angle. In this embodiment, a curved small-scale structure can be formed by applying droplets directly onto the surface of calligraphy paper or filter paper; alternatively, a material (e.g., wax) can be used to form a pattern (e.g., an array pattern) on the surface of the calligraphy paper or filter paper. Droplets can then be applied to the non-patterned regions of the calligraphy paper or filter paper to form a curved small-scale structure with a contact angle. FIG. 20 shows that the contact angle of the curved small-scale structure is 71.51 degrees and that the substrate of the curved small-scale structure has a width of 12.525 mm. For example, the biomedical test droplets can be applied to calligraphy paper to obtain a biomedical test specimen for a diagnostic device. The invention uses hydrophilic calligraphy paper as the substrate so that the characteristics of the calligraphy paper itself enable a biomedical assay thereon. Therefore, the cost is lower than that of a conventional biomedical assay. In contrast, the invention reduces cost and increases stability and ease of handling by using the characteristics of the calligraphy paper itself to prepare biomedical test specimens. Therefore, inexpensive calligraphy paper is recommended for use as a substrate for a biomedical assay. Additionally, the biomedical test specimens have broad applications in cell tests, antibody tests or in biomaterials and associated applications.

The preferred embodiments described above are illustrations rather than limitations of the applications of the invention. For a person skilled in the art, the preferred embodiments described above are illustrations rather than limitations of the applications of the invention. The invention is intended to enable various modifications, and similar arrangements are included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures. 

What is claimed is:
 1. A method for fabricating small-scale, curved structures, comprising: providing a calligraphy paper or filter paper; placing liquid-phase material droplets on mentioned calligraphy paper or filter paper to form a desired pattern; and wherein mentioned desired pattern has a contact angle to form convex curved small-scale structures.
 2. The method of claim 1, wherein mentioned droplets are biomedical test droplets.
 3. The method of claim 1, wherein material of mentioned calligraphy paper is PVC (polyvinyl chloride).
 4. The method of claim 2, wherein pore size of mentioned calligraphy paper is from 3 to 30 microns and thickness of mentioned calligraphy paper is from 0.15 to 0.3 millimeter.
 5. The method of claim 4, wherein mentioned droplets are biomedical test droplets.
 6. The method of claim 1, further comprising a step of forming a patterned layer on mentioned calligraphy paper or filter paper prior to mentioned placing liquid-phase material droplets.
 7. The method of claim 6, wherein material of mentioned patterned layer comprises ink, carbon powder, toner or wax.
 8. The method of claim 7, wherein material of mentioned calligraphy paper is PVC (polyvinyl chloride),
 9. The method of claim 8, wherein pore size of mentioned calligraphy paper is from 3 to 30 microns and thickness of mentioned calligraphy paper is from 0.15 to 0.3 millimeter.
 10. The method of claim 9, wherein mentioned droplets are biomedical test droplets.
 11. The method of claim 1, further comprising a homogeneity test on mentioned desired pattern.
 12. The method of claim 11, wherein mentioned droplets are biomedical test droplets. 